Nitrogenase reaction represents a major source of the usable form of nitrogen that supports the existence of human population. As such, understanding how small building blocks are assembled into a functional nitrogenase entity is of significant relevance to human health. Using combined genetic, biochemical, spectroscopic and structural approaches, we propose to investigate how M-cluster, the active center of molybdenum nitrogenase, is assembled. Specifically, we will examine the formation of a Fe-S core of M-cluster, the maturation of Fe-S core into an M-cluster, and the transfer of M-cluster from the assembly site to its target location in great detail. Through our proposed studies, we expect to refine the biosynthetic pathway of M-cluster, which will provide crucial insights into th structural-functional relationship of nitrogenase and the general assembly mechanism of complex metal clusters in biological systems.